This invention relates to inductive devices for use on printed wiring boards. More particularly, this invention relates to new and improved inductors, couplers, transformers and the like formed from rolled flexible circuits for use as electronic components in printed wiring board applications.
Every conductor carrying current generates a magnetic field which surrounds the conductor. This fieid is proportional to the current and is strongest near the conductor. An inductor is a conductor formed in such a way as to miximize the strength of the magnetic field for a given volume. The most compact form is a coil. The coil must be wound in one direction only or the magnetic field created by current flowing clockwise will tend to cancel the field generated by the current flowing in the counter-clockwise turns of the coil. The inductance of the coil is proportional to the square of the number of turns and the square of the internal radius of the coil. The inductance of a coil can be greatly increased by winding it on a magnetically permeable (ferromagnetic) core such as iron or ferrite. Additionally, a permeable "U" connecting the two ends of the core can be used to form a complete magnetic circuit. The permeable material magnifies the magnetic field just as a material having a high dielectric constant magnifies the electric field in a capacitor.
Inductors are used in a number of applications. Generally they are used in electronic filters. This is due to the fact that inductors tend to block the passage of high frequency signals while allowing low frequency signals to pass freely. The higher the inductance, the lower the frequency at which this blocking action begins to occur (this is called the cutoff frequency).
Transformers are two or more inductors in the same magnetic circuit. In the simplest case, transformers take the form of two insulated coils each wound around a common core. The two coils are thus linked via the magnetic circuit even though there is no direct electrical connection. One of the coils is called the primary coil and the other is the secondary coil. A sinusoidal signal is applied to the primary coil, generating a time varying magnetic field in the permeable core. The magnetic field induces a sinusoidal signal of the same frequency in the secondary coil. If both coils had the same number of turns and the circuit were completely efficient, the signal out of the secondary coil would match the signal applied to the primary coil. However, a different phenomenon occurs when the number of turns in the primary and secondary coils differ. For example, if the secondary coils has twice as many turns as the primary coil, the voltage induced in the secondary coil will be twice that of the voltage inputted to the primary coil. Thus, the voltage has been stepped up by the transformer in a manner proportional to the ratio of the number of secondary turns divided by the number of primary turns. This is a major use of transformers, that is to step voltages up and down. The secondary coil can be tapped at various locations to pull off fractions of the total secondary voltage. Another use of transformers is isolation. The primary and secondary coils are electrically isolated yet signal transfer occurs. DC signals are blocked, only time varying signals are transferred.
Presently, in the field of printed wiring board circuitry design, coils, transformers and other inductive elements are not easily utilized and include many problems and deficiencies. A significant difficulty with currently available inductive devices is their typically large size, difficulty and high expense in manufacturing and the fact that they are not easily fabricated as low-profile, flat components. Also, prior art inductive devices are heavy and bulky due to the large amounts of wires used in the fabrication thereof.
As a consequence of these problems, inductors are generally not surface mountable on a printed wiring board. In some cases, inductors are produced as part of the printed wiring board's original circuitry, whether plated, thin-film or thick-film. Inductors produced in this way have the disadvantage of using large amounts of valuable board space (real estate).